Heretofore, it has been said that the bacteria which cause dental caries in human is Streptococcus mutans out of streptococci (oral streptococci) inhabiting in the oral cavity. However, it is considered nowadays that dental caries is caused by a group of bacteria generically called as mutans streptococci which is constituted by bacterial species distinguished from each other in the different serotypes a to h (Hamada, S. and H. D. Slade, 1980, Microbiological Review 44:331–384; Hirasawa, M. et al., 1980, Infect. Immun. 27:1003–1011; and Loesche, W. J., 1986, Microbiological Review 50:353–380). Oral streptococci classified into mutans streptococci include Streptococcus cricetus, Streptococcus rattus, Streptococcus mutans, Streptococcus sobrinus and Streptococcus downeii. 
Further, the recognition is recently being accepted that Streptococcus mutans and Streptococcus sobrinus out of the bacteria classified into mutans streptococcl play particularly important roles in dental caries. Particularly, in Europe, the significance of Streptococcus sobrinus attracts attention, and there are reports that caries risk in the case where both of Streptococcus mutans and Streptococcus sobrinus inhabit in the oral cavity becomes higher than that in the case of Streptococcus mutans alone (Babaahmady, K. G. et al., 1998, Caries Res. 32:51–58; Kohler, B. and S. Bjarnason, 1987, Community Dent. Oral Epidemiol. 15:332–335; and Lindquist, S. and C. G. Emilson, 1991, Caries Res. 25:146–152). However, the distribution of Streptococcus mutans and Streptococcus sobrinus in dental caries, and the relative significance thereof to dental caries have not been clarified.
In Japan, it has been said that mainly found out the clinical samples of dental caries (carious cavity, dental plaque and saliva) is Streptococcus mutans. However, it is now discussed by the researchers how many Streptococcus sobrinus inhabit the oral cavity, and, whether or not the presence of Streptococcus sobrinus is really important for caries risk.
Mitis Salivarius Agar (MS, manufactured by Difco) is conventionally used as a selective medium for streptococci in the oral cavity. A medium prepared by the addition of sucrose and an antibiotic bacitracin to the Mitis Salivarius Agar is used as a selective medium for mutans streptococci (MSB medium). However, it is known that in the MSB medium, the recovery of Streptococcus sobrinus is lower than that of Streptococcus mutans. Further, Streptococcus mutans and Streptococcus sobrinus are closely resemble to each other in their colony morphology and so on, therefore, unless a researcher is conversant with the colony morphology of these species, under the circumstances, he or she cannot distinguish these species inhabiting on the same plate medium from each other by observation with the naked eye.
Some methods for detection and identification of Streptococcus mutans and Streptococcus sobrinus individually were reported. In these methods, these two kinds of bacteria are separated out of the oral streptococci including these two kinds of bacteria by means of a selective medium capable of substantially selecting these two kinds of bacteria alone, followed by biochemical or serological tests to distinguish these two species from each other (Gold, O. G. et al., 1973, Archs. Oral Biol. 18:1357–1364; Hirasawa, M. et al., 1980, Infect. Immun. 27:697–699; Kimmel, L. and N. Tinanoff, 1991, Oral Microbiol. Immunol. 6:275–279; Linke, H. A. B., 1977, J. Clin. Microbiol. 5:604–609; Schaeken, M. J. M. et al., 1986, J. Dent. Res. 65:906–908; Takada, K. et al., 1984, Infect. Immun. 45:464–469; and Wada, W. G. et al., 1986, J. Med. Microbiol. 22:319–323), or by a DNA prove method to distinguish them (Igarashi, T. et al., 1996, Oral Microbiol. Immunol. 5:294–298; and Shiroza, T. et al., 1998, Oral Microbiol. Immunol. 13:11–16). Further, some selective media for Streptococcus mutans have been reported (Gold, O. G. et al., 1973, Archs. Oral Biol. 18:1357–1364; Kimmel, L. and N. Tinanoff, 1991, Oral Microbiol. Immunol. 6:275–279; Linke, H. A. B., 1977, J. Clin. Microbiol. 5:604–609; Schaeken, M. J. M. et al., 1986, J. Dent. Res. 65:906–908; and Wada, W. G. et al., 1986, J. Med. Microbiol. 22:319–323). However, up to the present date, no selective culture technique capable of isolating Streptococcus sobrinus alone has been reported. Furthermore, the difference in the susceptibility to antibiotics between the different serotypes of mutans streptococci has been reported (Little, W. A. et al., 1979, Antimicrob. Agents Chemother. 15:440–443). However, no antibiotic which has the possibility to select Streptococcus sobrinus alone has not yet found.